1. Field of the Invention
The present invention relates to an automatic focusing device of an optical head for reading information or writing information in an information recording medium such as a digital audio disk or a video disk and more particularly to an automatic focusing device employing an astigmatic optical system.
2. Description of the Prior Art
Recently, active development of optical video disk devices and optical audio disks for reproducing information recorded optically in information recording disks or for recording information in information recording disks by the use of a head using a laser light has been made. The head of such an optical disk device (referred to as "optical disk head" hereinafter) is designed to focus a condensed laser light correctly on the information recording track of a disk serving as an information recording medium. Accordingly, the optical disk head needs to be equipped with an automatic focusing device. There is a known system employing an astigmatic optical system for information detection and for focal position detection.
FIG. 1 is a schematic light passage diagram of an automatic focusing device of an optical disk head employing an astigmatic optical system, FIG. 2 is a perspective view of part of the light passage of FIG. 1 and FIG. 3 is a circuit diagram of a servo mechanism including a light detector and the following circuits. Referring to FIGS. 1, 2 and 3, there are shown a light source 1 such as a semiconductor laser, a light beam 2 emitted by the light source 1, an objective lens 3, an optical disk serving as an information recording medium, storing digital audio signals or video signals in tracks and placed in the vicinity of the focal point of the objective lens, a reflected light beam 5 reflected from a focal spot on the disk 4 and transmitted through the objective lens 3, a beam splitter 6 for separating the radiated light beam 2 and the reflected light beam 5, a cylindrical lens 7 serving as an optical element for causing the astigmatic aberration of the reflected light beam 5 and having a longitudinal axis x extending perpendicularly to the surface of the sheet and an axis y extending perpendicularly to the axis x. One of the axes x and y is arranged substantially in the track direction on the disk. A light detector 8 is divided into four sections 8a, 8b, 8c and 8d by division lines extending at an angle of 45.degree. with respect to the x-direction and y-direction respectively, a reflection beam spot 9 formed in the light detector 8 by the reflected light beam 5, a focal point detecting circuit 10 consisting of adders 11 and 12 and a differential amplifier 13, an information detecting circuit 14 formed of an adder, a focus actuator 15 for moving the objective lens along the optical axis, a lens driving circuit 16 which energizes the focus actuator 15 to drive the objective lens, and an information reproducing circuit 17, not shown.
The functions of the automatic focusing device will be described hereinafter. A laser beam 2 emitted by the light source 1 is focused by the objective lens 3 and forms a light spot on a track of the disk 4. The disk 4 has an irregular surface formed of minute pits. Information is formed in the form of those pits. As the disk 4 is rotated, the laser beam radiated on the surface of the disk 4 is modulated by the pits and is reflected.
The reflected light beam 5 is separated from the laser beam 2 by the beam splitter 6 and is converted into an astigmatic beam which is converged in a single direction, by the cylindrical lens 7. The cylindrical lens 7 does not have the function of lens in the x-direction, therefore, the reflected beam is focused on a point P by the objective lens 3, while in the y-direction, the reflected beam is focused on a point Q by the focusing function of the cylindrical lens 7. Accordingly, the form of light distribution of the astigmatic reflected beam at the point Q, at the point P and at an intermediate point V between the points P and Q is an ellipse elongated in the x-direction, an ellipse elongated in the y-direction and an ellipse elongated either in the x-direction or in the y-direction or a circle, respectively.
The four-section light detector 8 is located at a position where the form of the light distribution of the astigmatic reflected beam is a circle, namely, the intermediate position V, when the disk 4 is positioned at a position corresponding to the focal point of the objective lens 3 (this position will be referred to as "focused position" hereinafter). Accordingly, the form of the spot 9 of the reflected beam on the light detector 8 is a circle when the disk 4 is at the focused position, an ellipse elongated in the x-direction when the disk 4 is at a position displaced toward the objective lens 3 from the focused position and an ellipse elongated in the y-direction when the disk 4 is displaced from the focused position away from the objective lens 3.
When the spot 9 of the reflected beam is circular, the light receiving sections 8a, 8b, 8c and 8d receive light equally. When the spot 9 of the reflected light is an ellipse elongated in the x-direction, the light receiving sections 8a and 8c receive more light than the rest of the light receiving sections. When the spot 9 of the reflected beam is an ellipse elongated in the y-direction, the light receiving sections 8b and 8d receive more light than the rest of the light receiving sections.
Accordingly, the output signal of the differential amplifier 13, i.e., the output signal E.sub.f of the focal position detecting circuit 10, corresponding to the difference between the output signal given by the adder 11 corresponding to the sum of the light reception outputs of the light receiving sections 8a and 8c and the output signal given by the adder 12 corresponding to the sum of the light reception outputs of the light receiving sections 8b and 8d is zero when the disk 4 is at the focused position, a positive value when the objective lens 3 is moved toward the optical disk 4 from the focused position and the spot 9 of the reflected beam is an ellipse elongated in the x-direction, and a negative value when the objective lens 3 is moved from the focused position away from the optical disk 4 and the spot 9 is an ellipse elongated in the y-direction. The magnitude of the output signal of the focal position detecting circuit is approximately proportional to the deviation of the objective lens 3 from the focused position. Accordingly, the objective lens 3 is focused automatically by energizing the focus actuator 15 through the control of the lens driving circuit 16 by the output signal E.sub.f of the focal position detecting circuit 10 to move the objective lens 3 along the optical axis.
The information read out from the optical disk 4 and carried by the reflected beam 5 is taken out by an information detecting circuit 14 which receives the sum of the light reception output signals of the light receiving sections.
However, the conventional automatic focusing device as described hereinbefore has the following problems in its practical use. As shown more specifically in FIG. 4A showing the detail of the constitution of the four-section light detector 8 of FIG. 3, there is a blind zone 18 of a finite width between the light receiving sections 8a, 8b, 8c and 8d to separate those light receiving sections from each other. If the width of this blind zone 18 is too narrow, cross talk between those light receiving sections increases and the output signal corresponding to the calculated difference is reduced adversely. According to exemplary design data of the conventional automatic focusing device, the diameter of the spot 9 when the objective lens is at the focused position is about 100 .mu.m and the lower limit of the width of the blind zone is approximately 10 to 15 .mu.m. Furthermore, the energy of a portion of the flux of the incident light on the light detector 8 that falls on the blind zone 18 does not contribute at all either to the output of the focal position detecting circuit 10 or to the output of the information detecting circuit 14.
On the other hand, since the reflected beam 5 is a Gaussian beam as shown in FIG. 4B and the center O of the reflected beam 5, where the flux density of the reflected beam 5 is the highest, coincides with the central portion of the light detector 8 where the blind zone is extending, the focused state detecting sensitivity is relatively low as compared with the sensitivity of detection in an ideal condition where the width of the blind zone is zero and cross talk does not occur. Still further, the loss of the incident energy due to the presence of the blind zone causes the reduction in the output of the information detecting circuit 14, which entails the deterioration of the SN ratio.